This invention relates to polybutylene modified masterbatches used in the preparation of impact resistant polypropylene with enhanced properties. More particularly, a masterbatch comprising an elastomer, a butene-1 polymer and an optional co-modifier affords a polypropylene with improved properties, notably impact resistance, elongation properties and weld line strength.
High impact polypropylene (HIPP) is conventionally produced by blending into polypropylene an impact blend consisting of a rubber such as an ethylene-propylene-diolefin elastomer (EPDM) and a co-modifier such as high density polyethylene (HDPE). The impact blend is typically "let down" into polypropylene by mixing pellets of the blend and polypropylene and extruding the mixture. While the addition of a rubber generally results in improved impact strength, the resulting HIPP may exhibit loss of weld line strength or ductility. Weld lines occur in injection molded articles in which flow fronts of injected polymer meet in the mold, either because polymer is injected through multiple gates or because injected polymer flow is parted, as by an insert in the mold. The line formed where the separate portions of polymer meet in the mold is the weld line. Ideally this is not perceptible and has the same strength as the remainder of the article. In the case of polymer compositions whose impact is improved by added elastomers, the weld line may be significantly weaker. It has now been discovered that substitution of a butene-1 polymer for all or portion of the comodifier used with the rubber in the impact blend, affords HIPP while avoiding these undesirable property changes in the finished product.
U.S. Pat. No. 4,078,020 discloses a thermoplastic composition comprising polypropylene, polybutylene and from 20 to 80% by weight of an elastomeric material such as an EPDM. The three components are blended to flux in a mixer; a masterbatch technique is not used. The composition has a higher rubber content and very low stiffness values, and does not exhibit the enhanced weld strength of HIPP prepared according to the present invention.
U.S. Pat. No. 3,455,871 discloses a masterbatch technique where butene-1 polymers are used as carriers to disperse additives in other polymers. The additives are described as non-olefinic materials such as pigments and lubricants, as opposed to the rubber or rubber/co-modifier mixed with the butene-1 polymer in the masterbatches herein.